Difference between revisions of "Corrosion rate calculation FAQ's"

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'''Does the corrosion rate model include the effects of FeCO<sub>3</sup>/FeS scales formation in the corrosion rate calculation?'''  
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'''Does the corrosion rate model include the effects of FeCO3/FeS scales formation in the corrosion rate calculation?'''  
  
The corrosion rate model includes the effect of FeCO<sub>3</sup> and FeS scales. These scales form on the surface of steel depending on environmental conditions. They are not passive films because they do not give rise to an active-passive transition (which results from the formation of oxide or non-stoichiometric oxide/hydroxide layers). However, they do affect corrosion rates by forming surface barriers and affecting both the anodic and cathodic dissolution in the active state. Their effect is most important for carbon steel and less important for corrosion-resistant alloys. A description of how the scales are modeled can be found in more detail in the paper below:
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The corrosion rate model includes the effect of FeCO3 and FeS scales. These scales form on the surface of steel depending on environmental conditions. They are not passive films because they do not give rise to an active-passive transition (which results from the formation of oxide or non-stoichiometric oxide/hydroxide layers). However, they do affect corrosion rates by forming surface barriers and affecting both the anodic and cathodic dissolution in the active state. Their effect is most important for carbon steel and less important for corrosion-resistant alloys. A description of how the scales are modeled can be found in more detail in the paper below:
  
 
[[media:General corrosion in CO2-H2S.pdf | A. Anderko and R. D. Young, CORROSION/99, Paper 31  ]]
 
[[media:General corrosion in CO2-H2S.pdf | A. Anderko and R. D. Young, CORROSION/99, Paper 31  ]]

Revision as of 09:57, 10 January 2018

Are the effects of a passivation layer included in the calculation of general corrosion rate model?

Passivation is included in the general corrosion rate model. For a description of how the passive layer is modeled, further information can be found in the paper: “Computation of Rates of General Corrosion Using Electrochemical and Thermodynamic Models” by Anderko et al., under the section “Active-Passive Transition”, see reference below. Additionally, the applications section describes how this model reproduces selected experimental data.

A. Anderko, P. McKenzie, and R. D. Young, CORROSION 2001 57:3, 202-213


Does the corrosion rate model include the effects of FeCO3/FeS scales formation in the corrosion rate calculation?

The corrosion rate model includes the effect of FeCO3 and FeS scales. These scales form on the surface of steel depending on environmental conditions. They are not passive films because they do not give rise to an active-passive transition (which results from the formation of oxide or non-stoichiometric oxide/hydroxide layers). However, they do affect corrosion rates by forming surface barriers and affecting both the anodic and cathodic dissolution in the active state. Their effect is most important for carbon steel and less important for corrosion-resistant alloys. A description of how the scales are modeled can be found in more detail in the paper below:

A. Anderko and R. D. Young, CORROSION/99, Paper 31


Does the corrosion rate model include the effects of other mineral scales, such as dolomite, minnesotaite, etc. in the corrosion rate calculation? No, the general corrosion model includes only the effect of the scales that form as a result of electrochemical reactions between the metal and dissolved CO2 or H2S. The carbonate, sulfate or silicate scales are not included. It would be very difficult if not impossible to model the effect of such scales because their effect depends of the rate of deposition, history of formation, adherence, etc. for which there is no suitable quantitative model. The additional scales should be considered as factors that can reduce the corrosion rate, but the extent of the reduction cannot be predicted.


Under determined conditions, the Pourbaix diagram indicates that a passivation layer of Minnesotaite will form (theoretically), yet the corrosion analysis output presents a significantly high corrosion rate (0.3-0.6 mm/year). What could be the reasons for this? Does the software assume/knows that the passivation layer formation is kinetically slow and does not include its effects?

The possible effect of minnesotaite is not included in the model for the prediction of corrosion rates. The thermodynamic prediction of the stability of this species should be considered as an indicator that the rate can be further reduced but there is no theory that would allow us to quantify such a reduction.


If a case involves a specific carbon steel alloy, for example L-80, which is not included in the OLI material database, what can be done to predict the corrosion rates in the OLI Corrosion Analyzer Software? For steel L-80 and similar carbon steels, the generic carbon steel should be used. In general, it is rather difficult to quantify the differences between various carbon steel grades. However, the electrochemical mechanisms are the same. Some parameters (such as exchange current densities) may differ on various carbon steel surfaces but a consistent quantification of these differences would not be practical in view of the available literature database.


Is it possible to manually add alloys for corrosion rate simulations?

It is possible, but no facility is provided for a user to do it. This is done internally at OLI. For example, in the case of L-80, we recommend using the generic carbon steels as this should be a good approximation.


Editor: Diana Miller, Author: Andre Anderko